The ability of the human eye to adjust itself for varying object distances is known as accommodation. As the human eye ages, the ability to adjust decreases. There then exists, for the ageing eye, a range of indistinct vision for near and intermediate tasks which increases with age and with decreasing accommodation.
The most common means devised to decrease or eliminate this range of indistinct vision are reading lenses, bifocal lenses, trifocal lenses, and progressive lenses.
Reading lenses have a single dioptric power to assist the eye to provide a range of clear vision for near tasks. This range of clear vision decreases with age and the range of distinct vision recedes so that even when the spectacles are removed there is a range of intermediate distance where vision is indistinct.
Bifocal lenses are provided when the wearer requires correction of distance vision as well as assistance for near tasks. In a bifocal lens, two separate zones of different dioptric focal power are provided. The range within which objects can be seen clearly through the distance vision zone does not overlap with the range within which the objects can be seem clearly through the near vision zone when the wearer is above the age of 45 to 50 years, even when a major part of the available accommodation is used.
Trifocal lenses have a third zone of a dioptric power intermediate between that of the distance vision zone and that of the near vision zone, interposed between those zones to extend into the intermediate region the range over which objects can be clearly seen. However, for wearers over about 60 years of age there is still a considerable range of indistinct vision between the near limit of the distance vision range and the far limit of the range of the intermediate zone.
The problem to be solved stems from the lack of a continuous range of vision for all distances. One solution for this has been the development of progressive lenses as described for example in U.S. Pat. No. 2,869,422. In these progressive lenses an intermediate zone is interposed between the distance vision zone and the near vision zone and progresses smoothly in increasing dioptric powers from that of the distance vision zone to that of the near vision zone. However, because of the kind of aspheric surfaces associated with them, all progressive lenses contain inherently unavoidable and unwanted astigmatism and distortion particularly in the peripheral portion of the intermediate transition zone.
This unwanted astigmatism causes a waving or rocking effect when the wearer's head is moved or when objects move in relation to the wearer, and this form of progressive lens has been unacceptable to many potential wearers because the wearing of such lenses can produce nausea or disorientation. A further disadvantage of this form of progressive lens is that the width of field for vision through the intermediate zone and the vision zone is relatively narrow. This is a constraint to natural lateral vision and forces the wearer to direct his gaze through the central area of the intermediate zone and the near vision zone.
A further disadvantage of progressive lenses is that the rate of change of dioptric power along the eye path or centre-line of the corridor along which the line of vision moves, is often so great that the power change over the aperture of the pupil causes a blurred image to be seen by the wearer when viewing objects at intermediate distances.
The development of progressive power lenses has been described by A. G. Bennett in a series of articles in "The Optician" (October, November 1970, February and March 1971).
Attempts have been made by designers of progressive lenses to reduce the degree or effect of the unwanted astigmatism. For example the lenses described in GB-A-2,056,106, GB-A-2,069,714, and U.S. Pat. No. 4,056,311 concentrate the astigmatism into relatively unused portions of the lens. Other designers have attempted to spread the astigmatism over a larger portion of the lens, thus decreasing the maximum value of astigmatism, as in the comparative examples of U.S. Pat. No. 4,315,673. However, it has not been possible to avoid the image degrading effects caused by the degree or distribution of this unwanted astigmatism.
GB-A-790,310 describes an ophthalmic lens whose lower portion forms a progressive surface which may blend with an upper distance vision zone of constant power, or may be cut locally to form a progressive surface of reduced area.
It is a main object of the invention to provide a multifocal lens which provides clear vision for intermediate distances up to the full range between the distance vision an near vision ranges, but which has considerably lower levels of astigmatism and distortion than conventional progressive lenses, and in which the eye path width and reading zone widths can be made significantly greater than with conventional progressive lenses.